remove old log.hasContents method
remove debug and status client support

* remove statusgui, statuslog, and debugclient commands
* deprecate debugPassword global config
* remove debug PB service (DebugService still handles the manhole)
* try scheduler now implements internally the subset of remote status objects
  that the tryclient expects internally
* PBListener is deprecated, although it will still work in configs
* master/buildbot/status/client.py is almost completely gone, except for
  PBListener
* data API fixes / hacks:
  * mq.startConsuming needs strings
  * change build.buildid.update -> finished
  * add startConsuming for StepsEndpoint
  * step['started_at'] can be None if the step hasn't started
  * step update methods now send events
  * mq message verification is disabled, since it is incompatible with sending
    multiple messages per event
  * builds' status strings aren't equivalent to the existing functionality